In electrophotography, a heat roller fixing method has been widely used to fix toner onto a recording material in view of its heat efficiency or suitability to high speed fixing. This heat roller fixing method is a method which passes a recording material having a toner image thereon between a heat roller with a heat source therein and a pressure applying roller, fusibly fixes the toner onto the recording material. In this method, although the method has the above advantages, there is the so-called "offset phenomenon" disadvantage in which toner on a recording material is transferred to a heat roller, and after the rotation of the heat roller the toner is transferred to undesired portions of another recording material and fixed, resulting in staining on the recording material.
In order to prevent the offset phenomenon of the fixing roller, a technique supplying a dimethyl silicone oil onto the surface of the heat fixing roller has so far been proposed.
This method effectively prevents the offset phenomenon, however, it requires a silicone oil supplying device and has further a problem of smoking. On the other hand, there are many proposals which give anti-offset property to the toner itself, for example, a technique using a resin having a broad molecular weight distribution as a binder resin in the toner as disclosed in Japanese Patent O.P.I. Publication No. 50-234652/1975 and a technique adding a releasing agent such as a low molecular weight polypropylene as disclosed in Japanese Patent O.P.I. Publication No. 49-65231/1974.
The above proposed techniques definitely have an excellent effect on the prevention of the offset phenomenon, however, there is the problem in that they have only a narrow fixing temperature range and do not sufficiently show a fixing property, resulting in reduced life time of the fixing roller due to staining. The conventional silicone oil has a positive-charging property as a frictional electrification property, and the fixing roller having the silicone oil on the surface is likely to be positively charged. Therefore, when toner having a negative-charging property is used, a phenomenon is likely to occur in which a recording material is wound around the fixing roller, and when the conventional silicone oil is applied to the fixing roller, there is a limitation to toner used.
There has recently been proposed a technique in which a stationary heated member, in place of the heat fixing roller, contacts a pressure applying roller through a moving film material, and a recording material with a toner image is passed between the roller and the film to fusibly fix the toner, as disclosed in Japanese Patent O.P.I. Publication No. 5-119530/1993. This fixing method has an excellent heat efficiency, but has the same disadvantages as the above heat roller fixing method.